The present invention relates to a feedback mechanism and a valve positioner having the feedback mechanism.
Generally, as shown in Japanese Patent Laid-Open No. 4-185902 (reference 1), a conventional valve positioner has an electrical/pneumatic signal converter for converting an input signal (electrical signal) into a pneumatic signal, an amplifier for amplifying the converted pneumatic signal and outputting it as an output pneumatic pressure to an operation unit for a valve, and a feedback mechanism for converting the actual working amount of the valve and outputting a feedback signal. The valve is driven to control such that the difference between the feedback signal and input signal becomes zero.
As another conventional feedback mechanism, one disclosed in Japanese Patent Laid-Open No. 11-125201 (reference 2) is known. As shown in FIG. 8, in the feedback mechanism disclosed in reference 2, a slot 13 is formed in a feedback lever 9 formed of a metal plate, and a connecting pin (feedback pin) 12 projecting on an actuating shaft 4 of the valve is inserted in the slot 13 to be movable in the longitudinal direction of the slot 13. When the connecting pin 12 is urged by a pin pressing spring 14 against one side wall 13a of the slot 13, it is prevented from loosely moving perpendicularly to the longitudinal direction of the slot 13.
In the feedback mechanism of the conventional valve positioner described above, when the actuating shaft 4 of the valve vibrates due to the influence of the fluid flowing in the valve main body, the force caused by the vibration is received by a contact portion A of the connecting pin 12 and feedback lever 9 and a contact portion B of the connecting pin 12 and pin pressing spring 14. Generally, when controlling the opening of the valve, the valve is often controlled to maintain a predetermined angle. Thus, the connecting pin 12, feedback lever 9, and pin pressing spring 14 keep in contact with each other at substantially the same points (contact portions A and B), and the force caused by the vibration concentratedly acts on only the contact portions A and B. As a result, the contact portions A and B may wear or be damaged.
If the connecting pin 12 wears, the diameter of the connecting pin 12 itself reduces to enlarge the gap with respect to the slot 13, and the actual working amount of the actuating shaft 4 cannot be detected at high precision. In the worst case, as the strength decreases, the connecting pin 12 may be damaged. If the feedback lever 9 wears, a recess is formed on that portion of the side wall 13a of the slot 13 with which the connecting pin 12 is in contact. When the connecting pin 12 enters the recess, the actuating shaft 4 and feedback lever 9 cannot operate smoothly. Accordingly, in this case as well, the actual working amount of the actuating shaft 4 cannot be detected at high precision. Also, when the strength decreases due to the wear, the feedback lever 9 itself may be damaged.
If the pin pressing spring 14 wears, it may be damaged in the same manner as the connecting pin 12 and feedback lever 9.
Therefore, the components must be inspected every predetermined period of time or when necessary, and components that have worn out must be exchanged, leading to cumbersome exchange operation. In the exchange operation, the plant and process related to the valve as the exchange target must be temporarily stopped.
It is an object of the present invention to provide a feedback mechanism and valve positioner in which wear and damage of the components are prevented to improve the reliability and to prolong the service life.
In order to achieve the above object, according to the present invention, there is provided a feedback mechanism comprising a feedback pin which displaces interlocked with a motion of a driving shaft of a valve that opens and closes, a feedback shaft which pivots interlocked with displacement of the feedback pin, a feedback sensor which outputs a feedback signal, to be used for opening/closing the valve, in accordance with a pivoting amount of the feedback shaft, and a connecting member which connects the feedback pin and feedback shaft, the connecting member having a cylindrical first bearing through which the feedback pin is inserted and a cylindrical second bearing through which the feedback shaft is inserted.